Conventionally, there is known a lens (so-called a Fresnel lens) having a lens surface that is divided to a plurality of concentric areas and formed in a saw blade shape (hereinafter called a Fresnel shape) as a lens suitable for reducing thickness and weight. This kind of lens is applied to a use in which a reduction of thickness is particularly advantageous and to a use in which an influence of diffraction can be ignored (for example, a magnifier, and an illumination system) and the like.
When this kind of lens is assembled into a product for illumination, a light source such as an LED (light emitting diode) and the like is fixed to a surface of a lens formed in the Fresnel shape after the center axis of light emitted from the light source is coaxially aligned with an optical axis of the lens.
Further, a Fresnel shape of this kind of lens includes a type having only a refraction surface for refracting light emitted from a light source and a type including a reflection surface in addition to the refraction surface. The latter type (the Fresnel shape including the refraction surface and the reflection surface) can capture and converge light emitted from a light source (for example, LED) at a large extending angle more efficiently than the former type (the Fresnel shape including only the refraction surface).
However, in Fresnel shape including the reflection surface, since a variation of an emission angle to a variation of an incidence angle on a reflection surface is larger than on the refraction surface, the variation of the incidence angle significantly influences a light distribution of light emitted from a lens, that is, a distribution of brightness of emitted light.
Therefore, when an incidence angle of light on a Fresnel-shaped reflection surface is offset from a designed incidence angle because, for example, an attachment error between a lens and a light source (an axis offset by which the center axis of light emitted from the light source is offset in a vertical direction to an optical axis of the lens) occurs, an emission angle of light on the reflection surface, that is, a reflection angle is significantly offset from a designed emission angle (reflection angle).
In the case, since a light distribution of light emitted from the lens is significantly offset from a designed light distribution, a problem arises in that satisfactory optical characteristics cannot be obtained.
As a method of solving the problem, it is contemplated to increase the number of projecting sections having a saw-blade-shaped cross section in a Fresnel shape as shown in, for example, patent literature 1. According to the method of the conventional art, an offset of light distribution due to an axis offset can be suppressed to some extent.